1. Field of the Invention
This invention relates to apparatus for positioning a prosthetic acetabular cup within an acetabulum during total hip replacement surgery.
2. Description of the Prior Art
As part of total hip replacement surgery, a prosthetic acetabular cup is implanted within the human acetabulum to substitute for the socket of the human hip joint. This is usually done to compensate for severe damage of the acetabulum due to disease, trauma, or other factors. A prosthetic femoral component is mated with the acetabular cup component to complete the total hip replacement surgery. In order to achieve optimal performance of the combined acetabular and femoral prostheses, the acetabular cup must be properly positioned in the acetabulum. An improperly positioned acetabular component can lead to dislocations of the hip joint, decreased range of motion, and eventual loosening or failure of both the acetabular and femoral components.
Interoperative placement of the acetabular component can be a surgically demanding task. The orthopedic surgeon must adequately fix the cup in the proper alignment while hampered by limited surgical exposure and relatively few clear reference points. In the past, the acetabular cup was provided in the form of an outer shell preassembled to a polyethylene bearing insert. Alternately, the cup shell and the bearing insert were provided as separate pieces to be assembled after the shell is affixed via bone screws or other means.
A wide variety of instruments are known in the art for inserting acetabular cup implants into position in the acetabulum. Many such insertion instruments include a first handle with a second handle extending therefrom to assist in the insertion procedure. Typically, the second handle is in a fixed orientation relative to the first handle. Examples of such acetabular instruments are disclosed in U.S. Pat. Nos. 4,528,980 to Kenna, 4,475,549 to Oh, 4,305,394 to Bertuch, Jr., 3,859,992 to Amstutz and 4,632,111 to Roche. Once the acetabular cup is attached to such instruments, the handles are in a fixed orientation with regard to the cup. With certain cup designs a particular orientation of the cup may be desirable to achieve proper anatomic positioning. An example of such a cup design is a cup which has pegs protruding from the outer surface, such as the pegged cup disclosed in U.S. Pat. No. 3,781,918 to Mathys. Accordingly, when the first and second handles are both in a permanently fixed orientation with regard to the cup, it may result in the second handle being in an awkward or inconvenient position in order to insert the cup in the desired orientation. Also, if the first handle needs to be rotated about its axis during the insertion of the cup or the removal of the insertion instrument from the cup, a fixed secondary handle may be awkward or get in the way.
Traditional cup positioners generally include at least a pusher ball sized and shaped to fit the recess within the acetabular component, a positioner arm, a positioner flange juxtaposed the pusher ball, and a pusher arm connected to the ball and to the flange to enable the user to push the prosthetic acetabular cup, when resting on the flange, into the prepared acetabulum. Generally, the cup must be manually held against the flange during implantation. In some cases, the cup is aligned on the flange by means of small projecting pins which can engage complementary holes in the face of the cup.
Separate cup pushers are often used in conjunction with a cup positioner to address some of these problems. The positioner is removed once the cup has been placed in the acetabulum, and the pusher is separately used to apply pressure to the cup.
In addition to the pusher ball, the flange, and the pusher arm, some acetabular cup positioning instruments include means for establishing reference angles to better insure proper alignment of the prosthetic cup. Many traditional positioner designs include a reference arm for establishing the abduction angle. U.S. Pat. No. 4,475,549 further includes means for establishing the anteversion angle.
The acetabular cup shell of the present invention has a shell with apertures for receiving bone screws and is usually not provided preassembled to a polyethylene bearing insert. The shell aperture pattern is asymmetrical with more holes being located in the superior quadrant of the shell than in the inferior quadrant (when placed in the acetabulum). As indicated above, the accurate positioning and orientation of the replacement shell is crucial for the restoration of adequate biomechanics during a total hip replacement procedure. For assuring this orientation, the implantation device is required to provide approximately 45.degree. abduction and 20.degree. anteversion, while firmly gripping the acetabular cup shell component during impaction and the subsequent release of the shell following final positioning. In the case of the present invention, the rod-like positioning instrument must also orient the superior hemisphere of the shell to the superior hemisphere of the acetabulum, assuring the accuracy of strategically placed screw holes which target adequate body masses of the anatomy and avoid the violation of critical vascular structures.
In the present invention, the positioning procedure just described is performed with the use of the molded, disposable plastic insert of the present invention which is coupled to each shell component at a specific orientation to the superiorly oriented aperture pattern thereon. The insert is adapted to receive a positioning rod in a manner specifically referencing the superior oriented screw hole pattern. The plastic insert can be engaged by the positioning rod only in an orientation consistent with the shell's superior screw hole pattern. This orientation is obtained by a central key or keyway on the rod's tip which is aligned with the rod's known alignment guide provisions (abduction/anteversion). The key or keyway engages a mating key element on the insert. Following positioning, impaction, and stability check, the rod/ plastic insert combination is removed from the shell by depressing the resilient spring or spring-like elements which disengage the undercut or groove of the shell component. The shell, being left intact, is now ready for continuation of the necessary procedure including screw application and bearing insert assembly. The retracted rod/plastic insert combination is then removed from the surgical site and the plastic insert of the present invention can be disengaged from the rod and disposed of.
The insert brings both simplicity and cost efficiency to modern total hip arthroplasty. By simply releasing the resilient spring elements, an efficient means of shell positioning and impaction is provided without the use of a bulky, multi-part positioning system commonly found in most hospitals. For hospital economics, extensive and costly instrumentation systems can be eliminated from increasingly tightened budgets. Both inexperienced and experienced support staffs are able to easily adapt to the simplified surgical procedure.